Hi Mina, I feel for you, what an awkward situation. If you don't feel the same, then firmly turn him down. If you feel the same then it's even more difficult! If so, perhaps as H said you can hold off until you are no longer his student. Not a good idea to pursue a relationship while he is supervising you, as Jenny said, the balance of power in the relationship is not equal. Take care and good luck.
I guess if you did want to start a romantic relationship you could think about transferring your supervision to somebody else? Think long and hard about this, I don't envy you, it's a very complicated situation. Just make sure you are not bullied into anything that is not 100% what you want. [I take it he's not married or anything? If he is, tell him where to go in no uncertain terms!] All the best.